Using a chronic fetal sheep preparation blood flow to the brain will be measured using a combination of total carotid flow determined with an electromagnetic flowmeter and distribution of flow using radio labeled nucleotides. The mechanisms controlling cerebral vascular flow will be investigated, namely, the effect of vagotomy on cerebral vascular flow and the effect of lowered oxygen tension. Finally, the effect of various steroids such as estrogen and progesterone as well as blocking agents for those compounds will be investigated.